1. Technical Field
The present invention generally relates to a multi-axle vehicle where one of the drive axles is driven by an electric motor. More particularly, it is related to the control of an all-wheel-drive vehicle's drive system.
2. Discussion
Four wheel drive systems in vehicles selectively or continuously communicate driving torque to primary (e.g., front) and/or secondary (e.g., rear) axles as well as bias torque between right and left wheels of an axle. Some conventional four wheel drive vehicles include an internal combustion engine that drives the primary axle and an electric motor that drives the secondary axle. These hybrid systems eliminate the need for a mechanical drive shaft extending between the primary and secondary axles thereby reducing the overall weight of the driveline and improving packaging options.
A four wheel drive system distributes engine power to the four wheels of a vehicle. The four wheel drive system uses two AC machines, a frequency variable generator (FVG) and an electric motor, to take power from the originally driven wheels, send the power from the originally driven wheels by electrical means and drive the originally un-driven wheels. The FVG generates the system power by converting the mechanical power to electrical power. The electric motor converts the electric power back to mechanical power. The mechanical power is used to deliver torque to enhance the automobile's tractive effort. The system transfers power from the engine to the rear wheels by first converting the mechanical power to electrical power using the FVG, and then transmitting the electrical power to the electric motor using electric power cables, and finally converting the electric power back to mechanical power using the electric motor.
Add-on, electric, all-wheel-drive schemes are becoming more predominant for original equipment manufacturers (OEMs), especially as the number of hybrid electric vehicles and electric vehicle considerations continue to grow. Generally speaking “all-wheel-drive” implies permanently engaged or automatically engaging four wheel drive. Different power electronics methods and apparatuses offer advantages from simplistic to sophisticated for the control of multi-phase power and torque delivery systems. These power and torque delivery systems typically consist of an electric generator connected, via phases, to an electric motor actuator or electric motor axle system which delivers the torque to the vehicle wheels.
Transmitting power by electrical means eliminates the mechanical shaft, which adds weight and is difficult to package. Another advantage of electric machine to electric machine systems is that they draw less power from the power drive electronics. The amount of power that is sent to the originally un-driven wheels can be controlled by the FVG. At a given FVG shaft speed, the amount of power transferred is controlled by electronically varying the FVG output power electrical frequency and magnitude. The magnitude of the FVG output power is a function of the magnitude of the excitation magnetic field, which in turn, is a function of the magnitude of the FVG rotor current. An inverter drives and controls the FVG rotor current by initially taking current from the vehicle battery and driving the FVG rotor winding.